1. Field of the Invention
The present invention relates to a method of and an apparatus for making window envelopes in which a clarifying agent is applied to a portion of paper making up an envelope to provide the envelope with a transparent window. The term “clarifying agent” is used herein to refer to a chemical agent which when applied to or carried in a part of paper acts to make that part of paper transparent.
2. Description of the Prior Art
While various types of the clarifying agent used to make a window portion of a paper envelope transparent have been proposed, it is desired that they be water-soluble together with the paper forming the envelope to allow such window envelopes after use to be recycled. For example, as described in JP H08-284097 A there is used a water-soluble clarifying agent in which 0 to 1.0 part by weight of silicone antifoaming or die-release agent and 0 to 20 parts by weight in active ingredient of a wax emulsion are added to 100 parts by weight in active ingredient of a resin and polymer solution obtained by neutralizing and making water-soluble 100 parts by weight in active ingredient of a mixture of carboxylated rosin resin and carboxylated acrylic polymer with 10 to 40 parts of ammonia water or one or a mixture of amines such as ethylenediamine or triethylamine or to 100 parts by weight in active ingredient of a mixture of that solution and 0 to 100 parts by weight in active ingredient of a solvent solution of styrene resin. Used as another water-soluble clarifying agent is sulfonated castor oil as described in JP H11-79190 A.
A clarifying agent to permit recycling must be water-soluble as mentioned above but as such is not much good in drying characteristics as those of UV curing type. Paper having a water-soluble clarifying agent applied thereto may, therefore, have to be passed over a roller while the clarifying agent has not enough been dried yet, and it is then very likely that parts of the undried solution upon contacting the roller are scraped off the paper, thereby mottling the window potion formed on the paper. Preventing this possibility requires the continuous paper carrying the clarifying agent to be slowed down in its rate of travel, but this then brings about inferiority in productivity of window envelopes.